development of commercial refrigeration systems with

Paper No. 192

DEVELOPMENT OF COMMERCIAL REFRIGERATION SYSTEMS WITH HEAT RECOVERY FOR SUPERMARKET BUILDING Armin Hafner(a), Stefano Poppi(b), Petter Nekså(a) , Silvia Minetto(c) and Trygve M. Eikevik(b) (a) (b) (c)

SINTEF Energy Research, 7465 Trondheim, Norway. [email protected]

NTNU, Kolbjørn Hejes v. 1B, 7491 Trondheim, Norway. Institute for Construction Technologies, CNR, 35127 Padova, Italy. [email protected]

ABSTRACT Supermarkets are commercial buildings with major energy consumption and contribute also to relatively large direct emissions of greenhouse (GHG) through emissions of refrigerants from the refrigeration plants and the air conditioning system installed. The huge majority of these systems, which are installed in European supermarkets, are applying HFC-404A as working fluid. Average annual leakage rates in Europe are in the range of 15-20 % of the total charge. Worldwide the figure is about 30 % and HCFC-22 being the main refrigerant in use. Restrictions on the use of synthetic refrigerants are coming into force in several countries. There is a need for a natural refrigerant which allows for a safe investment in efficient refrigeration systems which will not be forced to be retrofitted by legislation in the future. Systems applying R744 as the only refrigerant have been developed and more than 300 supermarkets exist in Europe, mainly in northern and mid-European countries. However, the systems still have large potential in development with respect to energy efficiency, heat recovery and cost efficiency. The paper describes the calculation method to identify the possible annual energy savings for different supermarket systems layout. The adoption of ejector and additional function such as heat recovery are evaluated. Results show relevant improvements in system efficiency when heat recovery has been adopted. Ejector usage is not diffused at the moment. Thus, theoretical analysis has been carried out. 1. INTRODUCTION Supermarkets do have a very high energy consumption, of 300-600 kWh/m2, compared to other commercial activities (office buildings ~150-200 kWh/m2, Norvdtvedt and Hafner (2012). In fact the electric energy used for heat ventilation and air conditioning (HVAC), hot water and refrigeration utilities covers about 4 % of the country total in United States and France and the 3% in Sweden, as reported in Orphelin et al. (1999) and Arias and Lundquist (2005). Improvements will therefore give significant energy savings. However, it is a challenging task because many factors are involved, e.g. ambient and indoor air temperature, relative humidity, building envelope, store opening time, people presence and many others. Many ideas have been introduced in order to reduce the energy consumption, however it is often difficult to evaluate the individual contribution for a complex system were the different measure interact with each other. Therefore a system approach must be taken, evaluating various parameters at the same time. In order to perform such an analysis and evaluation, a tool is needed in the form of user-friendly computer code for the evaluation of energy efficient measures in supermarkets and other premises related to the handling of refrigerated food. A model for a full supermarket has been developed and is presented together with result examples of its use. 2. SUPERMARKET MODEL In order to clearly identify the factors which influence the overall energy consumption of supermarkets and to identify possible savings, the supermarket system has been divided into several subsystems, as represented in Figure1. Each module and interactions between modules have been modelled by Poppi (2010). As case study, a 2125 m2 supermarket located in Trondheim (Norway) has been selected. It is a single floor building north-south oriented with 19,8 m2 windows. The store is assumed to be open from 8 am to 10 pm from Monday to Saturday. The retail area includes display cabinets for chilled (MT) and frozen (LT)

10th IIR Gustav Lorentzen Conference on Natural Refrigerants, Delft, The Netherlands, 2012 1

Paper No. 192

products, dry goods shelves and cold rooms. Details concerning cooling loads for cabinets and cold rooms are summarized in Table 1. Table 1 Cooling loads for cabinets and cold rooms MT MT

open vertical

Evap. Temp.(°C)

LT

LT MT

cold room cold room

MT

multideck open island semi-vertical

wall-site (+2°C)

(-22°C)

Rack-

Rack-

Rack-

Plug-in

Rack-

Rack-

system

system

system

units

system

system

-10

-35

-10

-10

-10

-35

1028

400

2263

845

Total Length (m)

33,35

22,5

6,8

12

Total HER (W)

34283

9000

15388

10140

Nominal HER (W/m)

Figure 1 Different modules in the supermarket model. 10411

2633

2.1 Outdoor climate Parameters such as air temperature, relative humidity, wind speed, solar radiation, height of the sun and solar azimuth are needed for the building thermal load calculations. In the specific simulation, data have been downloaded from METEONORM (Remund 2005) for Trondheim (Norway). METEONORM uses the average values from the period 1961-1990 for different weather stations according to the World Meteorological Institute (WMO) climate normal. 2.2 Building This module gives as an output the required heating and cooling of the building envelope. It considers convection through the building surfaces (walls, roof and ceiling) air infiltrations and internal heat loads, such as sensible and latent heat from people, lighting and other electrical loads, cabinets and cold rooms. Presence of people in the store has been assumed according to Hansen and Rasul (2008) adjusted for real logged typical total number of customers during a day, see Figure 2. Occupancy density is 0,84 person/m2.

Figure 2 Presence of people in supermarket model

2.3 Indoor climate Day-time and night time set point should be related to the outside air temperature in order to improve the total system efficiency. However, for simplicity the indoor air temperature has been set to 21°C at lower ambient temperature, but is allowed to increase if ambient temperature increases during summer period, as suggested by Nekså et al. (1998). Maximum allowed indoor temperature is 25 °C, when ambient temperature is higher than 27 °C. The relative humidity in the shop has been set to 55 %.


Paper No. 192

Indoor climate affects the performance of the cabinets and their energy consumption: the influence of the indoor thermohygrometical conditions on cabinets has been modelled according to Cecchinato et al. (2010). Air quality depends also on the concentration of carbon dioxide in the shop area. ASHRAE Standard 62.1 suggests a level of 1000 ppm; above this level fresh air should be provided. The CO2 level in the air of the shop area indicates the amount of fresh air that has to be supplied. Calculations show that ~2.53 10-6 m3/s per person of fresh air are needed to keep the maximum concentration of 1000 ppm constant. 2.4 Refrigeration system The refrigeration system currently installed in the supermarket is a CO2 DX cascade system, which is represented in Figure 3. Compressor rack systems, see 1 and 10 in Figure 3, have been sized using the total cooling load requested by cabinets and cold rooms: 77,5 and 2,6 kW respectively for MT and LT applications, plug-in units not included. The evaporation pressure has been fixed at 26 bar for MT utilities and 12 bar for LT utilities. Fin-coils heat exchangers have been considered in cabinets and cold rooms (8). The refrigeration side has been calculated considering isothermal conditions with 5K superheating at the evaporator and no subcooling at the condenser. The condensation of the LT circuit takes place in a plate heat exchanger (9); -7 °C is the condensing temperature. Excess heat is rejected in an air cooler (3), acting as a gas cooler or air cooled condenser depending on the working conditions. The optimised pressure of the gas cooler follows the saturation curve until 20 °C outlet temperature and then a linear trend has been fixed as shown in Figure 4. Air cooler condenser and gas cooler are sized when maximum heat load has been requested. 20 K and 10 K have been assumed respectively as t condenser (temperature difference between ambient air and condensing temperature) and gas cooler approach in these conditions. When lower heat load is required, dT has been reduced. According to the outside climate (input signal is received from the air temperature sensor), control system manages the 3-way plug valve (2): by pass is activated when air temperature is lower than a fixed set. Geothermal probes (4) could be considered as alternative to the air cooler.

Figure 3 Functional scheme of refrigeration system and the HVAC system

Norvdtvedt and Hafner (2012) show an other solution on how to integrate the refrigeration system with heating and ventilation. The total heat demand can be provided by the booster refrigeration rack with indirect heat rejection: water is sent to the floor heating circuits and to the heat exchangers of the HVAC unit. When additional cooling is required, MT compressors work as a reversible heat pump and produce heat. 10th IIR Gustav Lorentzen Conference on Natural Refrigerants, Delft, The Netherlands, 2012 3

Gascooler pressure [bar]

Paper No. 192

Gascooler outlet temperature [°C] Figure 4 Gascooler pressure versus outlet gas cooler temperature

Discharge gas temperature has been calculated estimating the isentropic efficiency in accordance with Chiarello et al. (2010). In order to evaluate the system efficiency, COPt (Total Coefficient of Performance) and EEH (Energy Efficiency for Heating) have been defined as follows:

(1)

1

2

(2)

“Qref” represents cooling loads from utilities, “Qr” the amount of the DHW production and heating amount recovered for HVAC system ,“Pcompres” the compression power consumption, “Php” the heat pump power consumption and “Q1 + Q2” the total heat demand from building. 2.5 HVAC system Air quality in the store is ensured by mixing two air flow rates: the first one coming from the ambient and the second one from the shop area. The measure of carbon dioxide pollution limits the amount of incoming fresh air. The recirculation flow rate maintains indoor climate conditions in the shop according to the set point. A functional scheme of the system is shown included in Figure 3 (referring to the yellow box). The refrigerant flows through a heat exchanger (HX1) when heating is requested by the HVAC system, otherwise heat is rejected to the ambient. HX2 provides cooling load if requested by the building envelope. A heat pump circuit (11) has been considered in order to provide heating when refrigerant plant can not cover the total request, while the refrigerant plant provides cooling. This heat pump circuit is described with red lines in Figure 3. When heat is not requested, the air cooler exchanger (3) provides to reject heat to the ambient. Heat pump capacity has been calculated when maximum heat demand is requested. Frequency speed regulator is installed for the heat pump compressor which guarantees fine regulation even during partial loads. Balanced ventilation with heat recovery has been assumed in order to reduce the energy consumption of the HVAC unit, utilising a rotary wheel which exchanges heat between the air coming from the shop and the air coming from ambient. The rotary wheel efficiency has been estimated: 0,85 and 0,9 respectively for sensible and latent load.


Paper No. 192

3. SIMULATION RESULTS 3.1 Building energy balance Simulations have been carried out in order to define the peak heat loads of the model supermarket during a day. Climate data have been downloaded from Meteonorm for 2nd of April in Trondheim, Norway. In order to estimate the amount of each contribution described in the section 2, following assumptions have been considered: 69,8 and 58,1 Watt respectively for sensible and latent heat for each person; 0,1 air change per hour for infiltration according to Haase et al. (2009); 15 and 3 Watt per square meters respectively for lighting and other loads such as computers and machinery. Heat extraction rates for cabinets and cold rooms have been assumed according to Table1. Figure 5 shows average loads hour by hour. Results are summarised in Table 2.

Figure 5 Heat loads profile in [kw] during 2nd April in Trondheim Table 2 Breakdown of the peak heat loads/total energy on the supermarket (2nd April) Heat loads

Day-time peak load (kW)

Night-time peak load (kW)

Total Energy (kWh/24h)

solar radiation

4

0

19

convection load

-90

-97

-1907

infiltration

-9

-10

-215

people

28

0

228

Internal loads

39

4

619

Cabinets & cold rooms

-27

-20

-579

As a literature example, Maidment and Tozer (2002) estimated a total peak heat load of approximately 507 kW and 420 kW for a 5000 m2 supermarket respectively during night and day time. 3.2 Energy saving with heat recovery Calculations have been carried out in order to define energy saving adopting heat recovery. Climate data have been downloaded from Meteonorm for the 2nd of April in Trondheim as an example. Two different scenarios have been made regarding how to manage the heat recovery of the refrigeration plant. In the first case (scenario 1), it is controlled to optimise the COP for the refrigeration side only, while in the second case (scenario 2) keeps high pressure at a fixed level of 95 bar to utilize maximum gas cooler heat for heating via the HVAC unit. Cooling load for HVAC is not taken into the investigation. The water has an inlet and outlet temperature of 15 °C and 65°C respectively; 10 dm3/h of water flow rate has been assumed for each person present in the supermarket. Figure 6 and Figure 8 show the daily heat loads profile and the daily energy consumption profile respectively for the second scenario. Even when gas cooler pressure has been set to 95 bar, it is necessary to cover the rest of the amount with an auxiliary heat pump system, which is described in Figure 3.


Paper No. 192 Table 3 Energy consumption: results of simulations for scenario 2 Heat loads

Day-time peak load (kW)

Nighttime peak load (kW)

Total Energy (kWh/24h)

Q1 + Q2

-85

-122

-1834

Heating

67

57

1250

Heat pump

18

65

585

Figure 6 Heat loads: daily profile for scenario 2

“Q1+Q2” need to be provided in order to balance the heat demand of the building. ”Heating” represent the air heating from the refrigeration plant. ”Heat pump” represents the heating load covered by the heat pump. Refrigeration plant and heat pump provide the total heating and cooling demands. In Table 3 simulation results have been summarised. In the first scenario the refrigeration plant works in order to optimize the efficiency of the refrigeration side only. This means that the compressor rack can work with lower pressure ratio than the second scenario and the compressors energy consumptions is reduced. The compression energy consumption and the gas cooler pressure set have been estimated in both scenarios and shown in Figure 7. Results are summarised in Table 4.

Figure 7 24h of power consumption and high side pressure level: scenario (1)

Figure 8 24h of power consumption and high side pressure level: scenario (2)


Paper No. 192 Table 4 Comparison between two scenarios Gas cooler pressure (bar)

Compression Power (kW)

Energy consumption for the Refrigeration System

Energy consumption for the Heat Pump system

Min

Max

Min

Max

kWh/24h

% of Tot.

kWh/24h % of Tot.

Scenario 1

40

44

8

11

234

17 %

256

Scenario 2

43

95

11

24

395

26 %

209

COPt

EEH

Min

Max

19 %

4

9,9

3,7

14 %

3,3

7,6

3,0

Scenario 1 requires 490 kWh/24h daily energy consumption (234 kWh/24h + 256 kWh/24h from ref. system and heat pump respectively) to cover 1834 kWh/24h heating demand. The EEH value is 3,7. Scenario 2 requires 604 kWh/24h daily energy consumption (395 kWh/24h + 209 kWh/24h from ref. system and heat pump respectively) to cover 1834 kWh/24h heating demand. The EEH value is 3,0. In order to reduce the investment costs, electrical heaters can be adopted instead of heat pump. Results of the investigation are summarised in Table 4b. Table 5 Comparison between two scenarios Gas cooler pressure (bar) Min

Max

Min

Max

Energy consumption for the Refrigeration System kWh/24h

Compression Power (kW)

Energy consumption for the Electrical heaters kWh/24h

EEH

Scenario 1

40

44

8

11

234

752

1,86

Scenario 2

43

95

11

24

395

585

1,87

EEH are almost the same for both scenarios. If no external heat is added into the refrigeration system, it is not able to cover the total heating demand. 3.3 Evaluation of the Energy consumption in different climate regions Simulations have been carried out in three different climatic regions: Trondheim, Frankfurt and Athens have been chosen as representatives of northern, central and southern European zones. Second scenario has been considered only, i.e. the same building was virtually moved to three different locations. Assumptions have been considered for the electrical power consumption of the rotary wheel heat exchanger, the water pump for the heat reclaim, the air cooler fans, the compressor rack of the heat pump, the internal loads of cold rooms, the lighting and the internal loads of the sales area, the lighting and fans of remote cabinets and plug-in cabinets. An hour by hour calculation has been performed; the high pressure was optimized for maximum heat recovery. The remaining heating demand has been provided by the heat pump. The investigation philosophy suggested in Arias J. et al. (2005) has been followed. “Annual energy demand per opening hours and total area” has been considered to compare the different layout of supermarkets. Results show that annual electricity energy demand in Trondheim is almost the same as for Frankfurt, while a similar shop in Athens would require 30 % more power, mainly due to increased air conditioning demand in the summer, see Table 5. Table 6 Annual electricity energy consumption TOTAL YEAR POWER CONSUMPTION 261

kWh/m2

0,056

kWh/(m2·h)

271

kWh/m2

0,057

kWh/(m2·h)

340 0,072

kWh/m2 kWh/(m2·h)

Northern Europe Central Europe Southern Europe

Arias J. et al. (2005) estimated appr. 0,125 kWh/(m2·h) for a typical 2000 m2 supermarket in Sweden. 10th IIR Gustav Lorentzen Conference on Natural Refrigerants, Delft, The Netherlands, 2012 7

Paper No. 192

4. EFFICIENCY IMPROVEMENTS: EJECTORS Two-phase ejectors utilise partly the expansion work available when high-pressure refrigerant is throttled in a motive nozzle inside an ejector. In the chosen lay-out, the ejector pumps the low-pressure mass flow rate of the evaporators to a higher pressure level into a separator. The kinetic energy of the motive flow rate is applied to accelerate the refrigerant flow downstream of the evaporators. In the mixing chamber of the ejector both fluid flows equalize their velocities which is transferred into a higher pressure level, compared to the suction pressure, inside the diffuser of the ejector. The primary fluid enters from the gas-cooler and the secondary fluid from the evaporator; at the ejector outlet the mixed flow converges to a liquid separator where the pressure value depends on the application. Elbel (2007), Drescher et al. (2007), Li and Groll (2005) and Yari (2009) report that ejectors may improve the system efficiency up to 15%. COPt has been calculated hour by hour in order to calculate the possible energy saving using ejectors instead of expansion device for the model supermarket. The efficiency analysis has been conducted for Trondheim, Frankfurt and Athens climates. Results of simulations carried out for southern regions are shown in Figure 9. Red and blue dots represent the COP and COPt respectively, i.e. one dot per simulated hour. The COP is lower than the COPt since it does not consider the heat recovery system. When the ambient air temperature increases, the gas cooler pressure increases. Thus, a larger amount of heat is available, which can be used to cover the heating demand from building. Heat recovery usage increases the COP. As summarized in Figure 9, higher efficiencies can be achieved when gas cooler pressure is close to 47 bar. Figure 9 COPt vs gas cooler pressure

Based on the periods of the year this condition occurs, new efficiencies have been estimated resulting from ejectors adoption. Results of Drescher et al. (2007), Li and Groll (2005) and Yari (2009) have been assumed as a basis. COPt has been calculated hour by hour and then up to 15% improvement in efficiency has been considered. The increasing efficiency reduces the compressors power consumption. Thus, the annual energy saving have been estimated compared to a traditional direct expansion system. Results from the calculations are summarized in Table 7. Table 7 Ejector power saving TOTAL YEAR

COMPRESSOR

TOTAL YEAR

COMPRESSOR RACK

TOTAL

COMPRESSOR

RACK POWER

POWER

POWER

ENERGY

ENERGY

CONSUMPTION*

CONSUMPTION **

CONSUMPTION**

SAVING

SAVING

kWh

kWh

kWh

kWh

556.287

133.889

555.071

132.673

0,22%

0,92%

576.668

149.114

569.425

141.871

1,26%

5,11%

723.039

206.847

701.807

185.615

2,94%

11,44%

POWER CONSUMPTION *

northern Europe central Europe southern Europe

*SOLUTION WITHOUT EJECTORS ; **SOLUTION WITH EJECTORS


Paper P No. 1922

Ejectors adooption increaases the efficciency of thee refrigeratio on plant. Thu us, annual ennergy saving g for both off refrigerationn and HVA AC systems can c be achieeved. As supported by calculations,, the compressor powerr consumptioon in south Europe E is ~4 40 % higherr than the northern part of Europe. The annual compressorr energy saving can be esstimated arou und 11 % in these region ns when applying ejectorss in the systeem as shownn in Figure 100. The novel ccommercial refrigeration r n system soluution includiing non-conttrolled ejectoors with diffe ferent ejectorr geometries allows applyying standarrdized ejectoors. The diffferent motivee nozzles alllow controlling the highh t oor load requirements. Th he MT-Comppressors are connected c too side pressurre according to ambient temperature the separatoor downstreaam of the ejeectors. The eejectors are applied a to maintain m a cerrtain pressurre differencee between thee separator and a the mediium pressuree receiver to be able to circulate c the liquid refrig gerant to thee evaporatorss. In case off reduce pressure lift caapability of the t ejectors, one of the MT-compreessors (rpm-controlled) can be connnected to thee vapour outllet of the meedium pressu ure receiver and thereby y reduces thee he ejectors inn operation to maintain a certain floow rate of refrigerant viaa entrainmentt ratio, whichh supports th the ejector iinto the sepaarator. This solution s secuures a constaant pressure difference d beetween the separator andd receiver

Figure 10 R R744 Booster System S with non-controllaable ejectors

Thus, COPt keeps relatiive high valu ues during a year in soutth Europe reegions compaared to otherrs due to thee ambient connditions.

5.. DISCUS SSION A computerr model has been develo oped to anallyse refrigeraation, air conditioning aand heating demands d forr supermarkeets in three European clim mate zones. IInteraction beetween the air a conditioniing unit and refrigerationn plant has beeen evaluatedd based on neew standardss and directiv ves for building performaance in EU. The annuall energy coonsumption for f three diifferent system layouts has been ccarried out. Trondheim,, Frankfurt aand Athens have been evaluated ass representaative of nortthern, centraal and south hern Europee climates. Reesults confirrm that climaate conditionns strongly in nfluence ann nual energy cconsumption ns. The samee buildings ennvelope has been b evaluatted, howeverr, highly diffferent energy y demands apppear when northern n andd southern cliimate zones have h been co ompared, i.e.. the annual energy consu umption is 5556 MWh in the northernn part of Euroope while 7223 MWh hav ve to be usedd in southern n Europe. Bo oth air condittioning and refrigerationn demand inccreased with ambient tem mperature. T The indoor teemperature in ncreases andd also the internal loads,, such as mettabolism heatt of people and a goods as well as coolling load of display d cabinnets and cold d rooms.

10th IIR Gustav Lorentzen n Conference oon Natural Reefrigerants, Deelft, The Nethe herlands, 2012 9

Paper No. 192

Heat recovery from the refrigeration plant and recovering energy from exhaust air reduces the annual energy demand in commercial refrigeration significantly. Many ‘R744 supermarkets’ applying heat recovery have been adopted this solution during last years. During cold seasons it is possible to use the heat reclaimed from the compressor rack systems to cover the heating demand instead of rejecting it to the ambient. The main profit is the increase of the entire system efficiency. Results of the simulation, which are carried out for north climate, show that only the refrigeration plant can cover up to 68 % heat demand for the HVAC system (scenario 2). The rest of the heating amount can be covered by a heat pump system which works independently. In this case, Frequency speed regulator should be installed for the heat pump compressor in order to guarantee a fine regulation even during partial loads. Thus, both the refrigeration and the heat pump system cover the total heating demand from building in many operating conditions. In order to reduce the investment costs, electrical auxiliary heat can be adopted. Finally the adoption of advanced components such as ejectors has been evaluated. Results show that an ejector application is most lucrative in the southern part of Europe, where more than 10 % of the energy could be saved, when implementing R744 ejectors.

6. CONCLUSION This work confirms that supermarket modelling is a complex topic since many parameters have to be considered. This kind of investigation, based on an hour by hour calculation is necessary in order to evaluate the implementation and integration of energy-saving technologies for high energy buildings like supermarkets. The computer model developed represents a tool to perform future studies about energy performance in supermarkets. Several sub modules are under development such as for a thermal active floor, heat rejection to energy well, advanced HVAC system, etc. Implementation of ejectors in supermarket refrigeration systems is recommended for installations which have to operate at high ambient temperature conditions. The proposed system solution allows the use of noncontrolled ejectors, which are able to operate at any ambient and load condition.

ACKNOWLEDGEMENTS This paper has been written as part of the work within the research project CREATIV, which is financially supported by the Research Council of Norway and several industry partners.

REFERENCES

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Cecchinato L. , Corradi M. , Minetto S. (2010). Energy performance of supermarket refrigeration and air conditioning integrated systems. Applied thermal engineering Chiarello M., Girotto S. , Minetto S. (2010). CO2 supermarket refrigeration system for hot climates. 9th IIR Gustav lorentzen conference of natural working fluids. Sydney. Drescher M. , Hafner A. , Jakobsen A. , Neksaa P. , Zha S. (2007). Experimental investigation of ejector for R-744 transcritical systems. Beijing: ICR07-B1-742. Elbel S. (2007) Experimental and analytical investigation of a two-phase ejector used for expansion work recovery in a trans-critical R744 air conditioning system. PhD Thesis at Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign. Haase M., Sartori I., Djuric N., HØseggen R. (2009). Simulation of energy-efficient office buildings in Norway. Eleventh International IBPSA Conference, (p. 1857-1864). Glasgow. Hansen M.and Rasul M.G. (2008). Performance assessment and improvement of an existing air conditioning System of a Supermarket: a case study on Bi-Lo supermarket. 3rd IASME/WSEAS Int. Conf. on Energy & Environment, (p. 55-60). Cambridge. Li D., Groll E.A. (2005). "Transcritical Co2 refrigeration cycle with ejector-expansion device". International Journal of refrigeration , Vol. 28, pp. 766-733 Lindeberg U. , Axell M. , Per Fahlén and Fransson N. (2007). Appropriate indoor climate for environmentally sustainable supermarkets- Measurements and questionnaires. Proceedings of Clima 2007 WellBeing Indoors . Maidment G.G.; Tozer R. M. (2002). Combined cooling heat and power in supermarket. Applied thermal Enginnering, 653-665. Nekså P. , Girotto S. , Schiefloe P.A. (1998). Commercial refrigeration using CO2 as refrigerant system design and experimental results. IIF-IIR Commission E2. Oslo. Norvdtvedt T.S. and Hafner A. (2012). Integration of refrigeration and HVAC in supermarkets. 10th IIR Gustav Lorentz Conference on Natural Refrigerants, (Paper No 184). Delft, The Netherlands. Orphelin M. , D. Marchio , D’Alanzo (1999). Are there optimum temperature and humidity set points for supermarket? Ashrae Transactions , CH 99-4-4. Poppi S. (2010). Development of commercial refrigeration systems. Master Thesis University of Padova Reinholt L. and Madsen C. (2010). Heat recovery on CO2 systems in supermarkets. 9th IIR Gustav Lorentzen Conference. Sydney. Remund J. (2005). Software METEONORM HYPERLINK http://www.meteonorm.com/pages/en/meteonorm.php Yari M.(2009). “Performance analysis and optimization of a new two-stage ejector-expansion transcritical CO2 refrigeration cycle “. International Journal of thermal science , 1997-2005.
